Oil refinery operations, in which crude oil is processed and naturally occurring sulfur and nitrogen compounds are removed from the crude oil, typically produce concentrated off-gas streams of hydrogen sulfide and ammonia. These concentrated tail gas streams may also be generated from processing natural gas and from coal, coke and biomass gasification operations. Oil refinery operations are increasingly faced with tail streams having increased contaminants, particularly sulfur and nitrogen, and there is increasing pressure on these operations to reduce the release of these contaminants into the environment and to provide purer refined products.
The off gas streams are typically further processed in one or more Sulfur Recovery Units (“SRU”) to recover the sulfur and to destroy the ammonia. A Claus SRU is the most common type of SRU, and are generally used as the main SRU to recover sulfur and to limit SO2 and other emissions. The Claus SRU off-gas is typically further treated in a Tail Gas Treating Unit (“TGTU”) to recover the residual un-recovered sulfur, which is generally required by environmental regulations to be used to limit SO2 and other emissions. The SCOT-TGTU process is typically used for this purpose, which involves thermal reduction of the Claus off-gas in order to convert the residual sulfur and sulfur compounds to hydrogen sulfide, and recycling it back to the Claus SRU for further sulfur recovery. Such further treatment in a SCOT-TGTU process is also needed to satisfy environmental regulations relating to the discharge of sulfur compounds into the environment. However, the SCOT-TGTU process is an energy- and process-intensive operation which very significantly adds to the cost of the overall sulfur recovery operation. It also reduces the total sulfur recovery capacity of the main Claus SRU due to the acid gas stream being recycled back to the Claus SRU, adding to the sulfur load.
Many processes have been developed for the recovery of sulfur dioxide in combustion gas streams to meet environmental requirements, but most require the addition of alkaline feed agents and generate waste products that are subject to additional disposal costs. For example, the process as described in U.S. Pat. No. 6,534,030 utilizes ammonium thiosulfate (ATS) as the absorption solution to absorb SO2, producing a sulfite rich ATS solution which is further contacted with a feed gas containing hydrogen sulfide and ammonium to form an ammonium thiosulfate containing solution. This sulfite-rich ATS mixture however, is not directly pH controlled with additional ammonia and not continuously re-circulated through the SO2 contactor with a separate re-circulating pump for maximum SO2 recovery efficiency as is the process presented in this patent. Further, control of ATS production volume is limited to the available ammonia in the feed gas stream containing hydrogen sulfide and the ammonium. The available ammonia limits ATS production volume and limits the amount of SO2 that can be absorbed. Also, control of the sulfur dioxide feed gas to match available ammonia and control absorption pH is problematic because of variations in upstream processes that ultimately generate the SO2 gas feed rate. The process is limited to the available NH3 in the H2S feed gas and must be controlled to meet the required molar ratios as shown in the well known overall reaction Equation 1:6NH3+4SO2+2H2S+H2O→3(NH4)2S2O3  Equation 1
Typically SWS gas contains approximately equal molar ratios, 1:1 of hydrogen sulfide to ammonia which is the same ratio of sulfur to ammonia in the ATS product. U.S. Pat. No. 6,534,030 describes that unabsorbed H2S from line 78 can be directed to an incinerator for combustion to produce sulfur dioxide or to a Claus SRU. The total amount of SO2 that can be absorbed and processed must be controlled to the SO2:NH3 molar ratio of 4:6. Any additional SO2 to the process can not be accommodated. Control of SO2 gas feed to the process through line 84 to maintain the required SO2:NH3 molar ratio is impractical when processing incinerated Claus SRU tail gasses. This is because both sulfur load to and sulfur recovery efficiency of the Claus SRU is highly variable and the incinerated Claus SRU tail gas will likely exceed the limited SO2 that can be accommodated by the limited ammonia feed from the SWS gas in this ATS process.
ATS solutions may be produced by the reaction of a solution of ammonium sulfite with elemental sulfur or with sulfides including hydrogen sulfide gas or sulfides or polysulfides in aqueous solution, as described in Kirk-Othmer Encyclopedia of Chemical Technology. The basic process involves absorption and reaction of SO2 with ammonia to produce an aqueous sulfite solution. The sulfur in this sulfite solution is ammonium sulfite (NH4)2SO3 or ammonium bisulfite NH4HSO3, and usually a mixture of both forming a pH buffering solution. The sulfite sulfur in solution is an oxidized form of sulfur having an oxidation valence state of S+4. This sulfite solution is then contacted and reacted with a reduced form of sulfur to produce ATS. The reduced sulfur can be elemental sulfur having a valence state of S0 or sulfide sulfur having a valence state of S−2 or polysulfide which contains a mixture of sulfur having valence states of S0 and S−2.
Many process variations utilizing different sources of sulfur and ammonia, different types of contacting and reacting equipment, different process flow schemes and a wide range of process conditions have been utilized for the production of ATS and many patents have been granted on these process variations. Most of these processes however are similar in that they use an ammonium sulfite solution as the primary SO2 gas absorption solution, which is then further reacted to produce ATS.